1. Field of the Invention
The present invention relates to an optical reflecting mirror such as an axial eccentric a spherical mirror and free curved surface mirror.
2. Description of the Related Art
Conventional optical reflecting mirrors having a curved reflecting surface, which is difficult to manufacture from glass, are formed by applying injection molding to a plastic material. However, molding using an injection molding method produces sinks, warpage, distortion, etc., on the optical reflecting mirror due to molding contraction, resulting in deteriorated accuracy of the mirror surface.
In molding using normal injection molding, the thickness from a mirror surface 100 to the back surface 101 partially varies as shown in FIG. 1(a), the molding contraction increases as the thickness increases, and it is therefore difficult to improve the accuracy of the mirror surface 100.
With regard to a method for forming an optical reflecting mirror whose thickness partially varies through injection molding, there is a proposal on a method (Japanese Patent Laid-Open No. H9-155928) which molds an optical reflecting mirror shaped in such a way as to connect a body having a mirror surface and a rib which intersects with this body through resin injection into a molding die, places a gate at a position facing the rib and preferentially sinks the rib while cooling and keeping the pressure of the resin so that sinks on the mirror surface can be suppressed and the mirror surface can be formed with a high degree of accuracy.
That is, this method is intended to complement the amount of contraction of the body having the mirror surface by sinking the side of the rib and prevent sinks from occurring on the mirror surface of the optical reflecting mirror.
However, the method described in Japanese Patent Laid-Open No. H9-155928 seems to be unable to form the mirror surface of an optical reflecting mirror having a complicated surface shape used for a projector (e.g., axial eccentric a spherical mirror and free curved surface mirror) with a high degree of accuracy.
On the other hand, the present inventor proposes a method for forming an optical reflecting mirror having a complicated surface shape such as an axial eccentric a spherical mirror and free curved surface mirror, capable of suppressing sinks on a mirror surface 100′ shown in FIG. 1(b) by equalizing the overall thickness from the mirror surface 100′ to the back surface 101′ (t1=t2=t3) and equalizing the overall amount of contraction (Japanese Patent Application No. 2003-134028).
However, while the conventional art can prevent sinks on the optical reflecting mirror, when an optical reflecting mirror having a mirror surface with nonuniform curvature such as an axial eccentric a spherical mirror and free curved surface mirror is formed through injection molding, there is a possibility of warpage or distortion occurring due to the shape of the mirror surface (difference in curvature) . That is, the optical reflecting mirror having nonuniform curvature of the mirror surface has a nonuniform sum total of vectors of the amount of contraction, causing warpage or distortion on the mirror surface and making it difficult to form a high accuracy mirror surface.